1. Field of the Invention
The present invention relates to a beam splitter and an observation apparatus.
2. Description of the Related Art
Microscopes in observation apparatuses include an optical system for magnifying and observing a part of an observation target (hereinafter referred to as a magnification observation optical system). Further, Japanese Unexamined Patent Application Publication No. 5-232385 and Japanese Unexamined Patent Application Publication No. 2002-148526, for example, disclose a microscope (device) includes an optical system for observing an entire observation target (hereinafter, referred to as an entire observation optical system) in addition to the magnification observation optical system. As such, by providing the magnification observation optical system and the entire observation optical system, both the magnification observation and the entire observation of the observation target can be performed by switching the optical systems.
In the microscope (device) according to Japanese Unexamined Patent Application Publication No. 5-232385 and Japanese Unexamined Patent Application Publication No. 2002-148526, since an optical axis of the magnification observation optical system and an optical axis of the entire observation optical system do not match each other, a stage needs to be moved when the magnification observation is performed and when the entire observation is performed, with respect to the same observation target. Thus, while the magnification observation is being performed, a field of view is narrow and thus it is difficult to perceive which spot in the observation target is currently being observed. Particularly, if a large number of wells are sequentially magnified and observed such as in the case where the observation target is for example, e.g., a micro plate for cell culture, an observer might lose the order thereof in the middle of the observation.
Thus, as illustrated in FIG. 14, for example, the optical axis of the entire observation optical system including a camera 3 and a lens 30 for entire observation and the optical axis of the magnification observation optical system including a camera 4 and a lens 40 for magnification observation can be matched each other by using a semitransparent mirror 2. In such an observation apparatus, the magnification observation can be performed while keeping track of the current observation spot by displaying a pair of images picked up substantially at the same time by the cameras, respectively, on a display, for example.
However, in the observation apparatus illustrated in FIG. 14, the magnification observation lens 40 such as a microscope objective lens cannot be brought closer to an observation target 9 than it is as described in FIG. 14 with a distance of D/2+α therefrom, where D denotes a dimension such as a length or a width of the observation target 9, and α denotes a distance with which the semitransparent mirror 2 and the magnification observation lens 40 do not interfere with each other. And if an entrance pupil diameter EP of the magnification observation lens 40 is approximated by an outer diameter of this lens, it can be expressed by α≈EP/2.
Thus, a (maximum) numerical aperture (NA) of the magnification observation lens 40 is expressed as follows:
                              NA          =                    ⁢                      n            ⁢                                                  ⁢                          0              ·              sin                        ⁢                                                  ⁢            θ                                                        ≈                    ⁢                                    (                              EP                /                2                            )                        /                          (                                                D                  /                  2                                +                α                            )                                                                    ≈                    ⁢                      EP            /                          (                              D                +                EP                            )                                             and NA is decreased in accordance with D, where n0 is a refractive index of air and n0≈1 is assumed, and θ is an aperture angle of the magnification observation lens 40 (angle of an entrance pupil of the lens with respect to an object point of the observation target 9 on the optical axis of the magnification observation optical system). Therefore, (theoretical) resolution δ (=0.61λ/NA) of the magnification observation lens 40 is increased in accordance with D (deteriorates), where λ is a wavelength of incident light.
Further, depending on D/2+α, the magnification observation lens 40 with a long operation distance (in the case of a microscope objective lens, a distance between its lens barrel front edge and the object point) is needed.